1. Field of the Invention
The present invention relates to a carrier which composes a two component type developer used to develop an electrostatic latent image or magnetic latent image in electrophotography or electrostatic printing, and a two component type developer making use of the carrier. More particularly, it relates to a carrier which composes a two component type developer remarkably improved in running performance and environmental properties, and a two component type developer making use of the carrier. The present invention also relates to an image forming method carried out using the two component type developer.
2. Related Background Art
As developing methods for electrophotography, there are a number of methods such as cascade development and magnetic brush development. Developers used in these developing methods include one-component type developers and two component type developers.
Carriers that composes such two component type developers can be roughly grouped into a conductive carrier and an insulative carrier. The conductive carrier is usually comprised of oxidized or unoxidized iron powder. Two component type developers comprised of this iron powder carrier, however, have the problem that the triboelectric chargeability to a toner is unstable and hence fog tends to occur on visible images formed. More specifically, as the two component type developer is used, toner particles adhere to and accumulate on the surfaces of the iron powder carrier particles (i.e., toner is spent), so that the electrical resistance of carrier particles increases to lower bias currents, and also to make the triboelectric chargeability unstable, resulting in a decrease in the image density of visible images formed and an increase of fog. Thus, when copies are continuously taken by an electrophotographic copying machine by the use of the two component type developer containing the iron powder carrier, the two component type developer may deteriorate upon copying on a small number of copy sheets and hence it becomes necessary to change the two component type developer at an early stage, resulting in a high cost after all.
Accordingly, in recent years, as a measure for eliminating disadvantages of such iron powder carrier, it is proposed to use ferrite particles as the carrier (Japanese Patent Applications Laid-open No. 52-565367, No. 58-202456, etc.).
Ferrite carriers hitherto put into practical use can exhibit excellent performance not achievable by iron powder carriers, when used in specific toners and electrophotographic equipments. However, taking account of various required performances, there is still room for further improvement, and no perfect ferrite carriers are known at present. For example, even if a carrier with a proper electrical resistance can be obtained by selecting ferrite composition and firing temperature, no desired magnetic properties can be obtained, or even if the electrical resistance and magnetic properties can be kept within proper ranges, the charge quantity can not be well controlled. Such difficulties have been involved.
The insulative carrier is commonly typified by a carrier comprising carrier core particles comprised of a ferromagnetic material such as iron, nickel or ferrite whose surfaces are uniformly coated with an insulating resin. Two component type developers that employ this carrier may little cause the melt-adhesion of toner particles to the carrier surfaces, compared with the case of the conductive carrier, and at the same time the triboelectric chargeability of carriers on toners can be controlled with ease. Hence, there is the advantage that it is suitable particularly for high-speed electrophotographic copying machines in view of its superior durability and long lifetime.
There are various performances required for the insulative carrier. Particularly important performances can be set out as proper chargeability, impact resistance, wear resistance, a good adhesion between cores and coating materials, and uniformity in charge distribution.
Moreover, in recent years, there is an increasing commercial demand for making copying machines achieve a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that color images can be formed in a high image quality. Making smaller the particle diameters of toner particles, however, results in an increase in the surface area per unit weight, tending not only to bring about an excessively large quantity of triboelectricity of the toner but also a decrease in charging speed. This is accompanied with a possibility of the insufficiency of image density or the deterioration of durability or running performance such as fog and toner scatter.
That is, in the development of electrostatic latent images held on an electrostatic latent image bearing member, toner is blended with a carrier formed of relatively large particles and is used as a two component type developer for electrophotography. The composition of both the toner and the carrier is selected so that as a result of their mutual contact friction the toner can have, e.g., a polarity reverse to the charges present on the photoconductive layer. As a result of contact friction between the both, the carrier further electrostatically attracts the toner to its particle surfaces to transport the toner as a developer through a developing assembly and also feed the toner onto the photoconductive layer of the electrostatic latent image bearing member.
When, however, copies are continuously taken on many copy sheets by an electrophotographic copying apparatus using such a two component type developer, although sharp images with a good image quality can be obtained at the initial stage, edge effect with much fog may seriously occur after copies have been taken on several tens of thousands of sheets, resulting in images having poor gradation and sharpness.
In color copying carried out using toners with chromatic colors, continuous gradation is an important factor that influences image quality, and the edge effect that stresses only margins of images, occurring after copies have been taken on a large number of copy sheets, greatly damages the gradation of images. For example, quasi-contours due to the edge effect are formed in the vicinity of actual contours, resulting in a loss of copying reproducibility including color reproducibility in color copying.
Image area used in conventional black and white copying is 10% or less and images are almost held by line images as in letters, documents, reports and so forth. On the other hand, in the case of color copying, image area is 20% at least, and images are held by gradational solid images at a reasonable frequency or occupancy as in photographs, catalogues, maps, pictures and so forth.
When copies are continuously taken using such originals having a large image area, reproductions with a high image density can be obtained at the initial stage in usual instances, but the feeding of toner to the two component type developer may become insufficient with time to cause a decrease in density, or the toner being fed and the carrier may mix in the state of charge insufficiency to cause fog or cause a local increase or decrease in toner concentration (which indicates toner-carrier mixing ratio) on the developing sleeve, tending to result in blurred images or non-uniform image density. This tendency becomes more remarkable when the toner has a smaller particle diameter.
Such under-development and fog are presumed to be caused by an excessively low toner content (i.e., toner concentration) in the two component type developer or a poor rise for rapid triboelectric charging between the toner being fed and the carrier contained in the two component type developer, where any uncontrollable, insufficiently charged toner thereby produced participates in development.
It is essential for color developers to have the ability to always output images with a good image quality in the continuous copying of originals having a large image area. To deal with originals having a large image area and requiring a very large toner consumption, measures hitherto taken have more relied on improvements of developing apparatus than improvements of developers themselves. That is, it has been attempted to increase the peripheral speed of a developing sleeve or make a developing sleeve have a larger diameter so that the developing sleeve can be brought into contact with electrostatic latent images more times.
Such measures can be effective for improving developability, but may greatly limit the lifetime of apparatus because of an in-machine contamination due to toner scatter occurring in developing assemblies or because of an overload on the drive of developing assemblies. In some instances, measures are also taken in which developers are put in developing assemblies in large quantities in order to compensate the insufficiency of developability of the developers. Such measures, however, cause an increase in weight of the whole copying machine, a cost increase due to the apparatus that must be made larger in size and an overload on the drive of developing assemblies as in the above case, and are not so much preferable.
Now, studies are reported on improvements made from both directions of toners and carriers for the purpose of maintaining a high image quality over a long period of running.
For the purpose of improving image quality, several developers are proposed. For example, Japanese Patent Application Laid-open No. 51-3244 discloses a non-magnetic toner in which its particle size distribution is controlled so that the image quality can be improved. This toner is mainly composed of toner particles having a particle diameter of 8 to 12 .mu.m, which are relatively coarse. According to studies made by the present inventors, it is difficult to "lay" the toner with such particle diameter onto latent images in a uniform and dense state, and also the toner, as having the feature that particles with a size of 5 .mu.m or smaller are in an amount of not more than 30% by number and particles with a size of 20 .mu.m or larger are in an amount of not more than 5% by number, tends to cause a lowering of uniformity because of a broadness of its particle size distribution. In order to form sharp images by the use of the toner comprised of such relatively coarse toner particles and having a broad particle size distribution, the toner particles must be thickly overlaid so that any spaces between toner particles can be filled up to increase apparent image density. This brings about the problem of an increase in the consumption of toner necessary to attain a given image density.
Japanese Patent Application Laid-open No. 54-72054 discloses a non-magnetic toner having a sharper particle size distribution than the above toner. This toner, however, contains medium-size particles with a size of as large as 8.5 to 11.5 .mu.m, and has room for further improvement for a toner with a high resolution.
Japanese Patent Application Laid-open No. 58-129437 discloses a non-magnetic toner having an average particle diameter of 6 to 10 .mu.m and held by particles with a size of 5 to 8 .mu.m in the greatest number. This toner, however, contains particles with a size of 5 .mu.m or smaller in an amount of as small as 15% by number, and tends to form images lacking in sharpness.
As a result of studies made by the present inventors, they have discovered that toner particles with a size of 5 .mu.m or smaller contribute the clear reproduction of contours of latent images and have a chief function of densely "laying" the toner onto the whole latent image. In particular, electrostatic latent images on a photosensitive member have a higher electric field intensity at their edges, the contours, than at their inner sides because of concentrated lines of electric force, and the quality of toner particles gathering at the contours influences the sharpness of image quality. The studies made by the present inventors have revealed that the control of the quantity of toner particles with a size of 5 .mu.m or smaller is effective for solving the problems concerning the sharpness of image quality.
Accordingly, the present inventors have proposed in Japanese Patent Application Laid-open No. 2-222966 a toner containing toner particles with a size of 5 .mu.m or smaller in an amount of 15 to 40% by number. This has brought about a reasonable improvement in image quality, but it is sought to achieve a more improved image quality.
Japanese Patent Application Laid-open No. 2-877 discloses a toner containing toner particles with a size of 5 .mu.m or smaller in an amount of 17 to 60% by number. This has certainly brought about stable image quality and image density, but it has been found that, when originals requiring a large toner consumption as in photograph originals are continuously copied, the particle size distribution of toner may change if measures are taken from the direction of toners only, making it difficult to obtain always stable images.
Meanwhile, Japanese Patent Applications Laid-open No. 51-3238, No. 58-144839 and No. 61-204646 suggest average particle diameter and particle size distribution of carriers. Of these, Japanese Patent Application Laid-open No. 51-3238 makes reference to a rough particle size distribution. It, however, has no specific disclosure as to magnetic properties closely concerned with developing performance of developers or transport performance thereof in developing apparatus. Moreover, as for the particle diameter of carrier, carriers used in Examples all contain particles with a size of 250 meshes or larger in an amount of as large as about 80% by weight or more and also have an average particle diameter of 60 .mu.m or larger.
Japanese Patent Application Laid-open No. 58-144839 only discloses average particle diameter of a carrier. It makes reference to the quantity of fine powder that influences the adhesion of carriers to photosensitive members and the quantity of coarse powder that influences the sharpness of images. It takes account of performance of color copying, and has no detailed disclosure as to particle size distribution of carriers.
Japanese Patent Application Laid-open No. 61-204646 discloses as the gist of the invention a combination of a copying machine with a suitable developer, and has no specific disclosure as to the particle size distribution or magnetic properties of carriers. It also has no disclosure as to why the developer is effective for the copying machine.
Japanese Patent Application Laid-open No. 49-70630 has a disclosure relating to magnetic force of carriers, which, however, is concerned with iron powders used as carrier materials, having a larger specific gravity than ferrites, also having a high saturation magnetization. Iron powder carriers have been hitherto put into wide use, but, because of their large specific gravity, tend to make the weight of copying machines larger or cause an overload on drive torque, and also have a large environmental dependence.
A ferrite carrier disclosed in Japanese Patent Application Laid-open No. 58-23032 concerns a porous material with many voids. Such a carrier tends to cause the edge effect, having a poor durability, and has been found to be unsuitable for color copy carriers.
It has long been sought to provide a developer that enables continuous reproduction of images with a large image area, using a developer in a small quantity, and can satisfy the performance specific to color copying that no edge effect may occur even after running. Studies are made on developers and carriers, almost all of which, however, are proposed taking account of black and white copying, and only a little of which are proposed as those applicable also to full-color copying. It is also sought to provide a carrier having the ability to continue reproduction of images having an image area of 20% or more, which are nearly solid images, and having the ability to decrease the edge effect and maintain the uniformity of image density on a sheet of reproduction.
Under such circumstances, as disclosed in Japanese Patent Application Laid-open No. 2-281280, a carrier with a narrow particle size distribution in which the presence of fine powder and the presence of coarse powder have been quantitatively controlled, is proposed to achieve a carrier improved in developing performance.
However, as previously stated, there is an increasing commercial demand for making copying machines have a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that a color image can be formed in a high image quality. Making smaller the particle diameters of toner particles results in an increase in the surface area per unit weight, tending to bring about an excessively large quantity of triboelectricity of the toner. This is accompanied with a possibility of the insufficiency of image density or the deterioration of running performance.
Thus, for the purpose of preventing the insufficiency of image density or the deterioration of running performance, caused by the toner made to have a smaller particle diameter, or for the purpose of improving development efficiency, it is attempted to make carrier particles have a smaller diameter. Such carriers, however, have achieved no quality high enough to stand against changes in the environment of toners or changes in the quantity of triboelectricity after running, and, under existing circumstances, it is difficult to achieve all the high image density, high image quality and good anti-fogging and prevention of carrier adhesion.